The present invention relates to an oscillator for a semiconductor integrated circuit.
A multivibrator circuit is known in the prior art as an oscillator circuit that alternately activates and inactivates two transistors. Further, for an oscillator circuit incorporating an emitter-coupled multivibrator, an oscillation control circuit has been proposed to output a stable oscillation frequency by reducing the influence of temperature dependency and variation between lots on the base-emitter potential difference (for example, refer to Japanese Laid-Open Patent Publication No. 2005-167752, FIG. 1). In the oscillator circuit described in Japanese Laid-Open Patent Publication No. 2005-167752, common current flows through transistors that determine the oscillation frequency. This suppresses the influence of the base-emitter voltage difference. Further, as for current determined by a resistor that controls the dynamic range or the like, an emitter follower is used to suppress the base-emitter voltage difference of transistors determined by this current.
Further, an emitter-coupled multivibrator oscillation circuit having a relatively small circuit scale and eliminating the need for an external amplification circuit, which consumes a large amount of power, has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 6-6187, FIG. 1). In the oscillator circuit described in Japanese Laid-Open Patent Publication No. 6-6187, an AC-coupling capacitor is connected to the output side of the oscillator circuit, and one or more amplification circuits, each formed by a self-biased CMOS inverter, are connected to the AC-coupled capacitor. The self-biased CMOS inverter includes two conductive MOSFETS, which are connected in series between a power supply and ground. The MOSFETs have gates connected to each other. A bias resistor is connected to a node between the MOSFETs. Further, the self-biased CMOS inverter has a gate area determined so that through current, which is generated when the two conductive MOSFETs are switched, is regulated to a predetermined value.
However, the swing of an oscillator circuit formed by a CMOS transistor is small. Particularly, a high frequency would affect the operation speed and parasitic capacitance of the transistor. As a result, the oscillation amplitude decreases in inverse proportion to the frequency. For example, in the oscillator circuit described in Japanese Laid-Open Patent Publication No. 2005-167752, the amplitude voltage is expressed by “R3*I2.” Thus, full swing cannot be performed and amplification is necessary. Accordingly, an amplifier for realizing full swing is necessary for the multivibrator. This results in the necessity for bias current. The oscillator circuit described in Japanese Laid-Open Patent Publication No. 6-6187 includes an amplification circuit and is thus capable of full swing. However, the amplification circuit increases power consumption. In this case, when the amplitude is small, the amplification gain and accuracy must be increased. This requires a larger amount of current.